Mechanism of Urine Formation - NEET Important Topic

Urine is a liquid or semi-solid solution of waste produced by humans and animals due to physiological activities and also contains harmful compounds produced by excretory organs during fluid circulation. The human body frequently excretes waste in the form of urine. In the excretion process, our kidneys play a crucial role.

Urine contains both organic and inorganic substances. Examples of organic substances in urine include urea, creatinine, and uric acid. Inorganic solutes include sodium, potassium, and chloride. The presence of proteins gives it an acidic pH of roughly 6. Urobilin is principally responsible for the urine's colour in healthy individuals. In turn, urobilin is a byproduct of the breakdown of heme from haemoglobin during the apoptosis of old blood cells. This article explains the mechanism of urine formation.

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Mechanism of Urine Formation 

Waste is often expelled by the human body in the form of urine, and urine is made in the nephron, the kidney's basic structural unit. Nephrons are important as they are the functional and structural units in the kidney. Nephrons, which filter the whole volume of blood in the body 60 times a day, are the backbone of the urinary system.

Glomerular filtration, reabsorption, and secretion are the three primary stages in the production of urine in our bodies.

Glomerular Filtration

• The glomerulus is where blood is filtered. The Bowman's capsule epithelium, the endothelium of glomerular blood vessels, and the membrane between these two layers - all participate in this form of filtration.

• Glomerular filtration begins when the fluid fraction of blood is filtered by the glomerulus and enters the glomerular capsule as glomerular filtrate. The filtrate is a liquid that enters the nephron after leaving the bloodstream. Except for the formed elements and plasma proteins, it encompasses all chemicals in the blood.

Filtration in Nephron

• Water, glucose, ions, amino acids, and nitrogenous wastes are all found in this filtrate, which is collected by the renal corpuscle.

• Water and other solutes are forced out of the circulation into Bowman's capsule by high blood pressure in the glomerulus. This high blood pressure is due to the afferent arteriole being more prominent than the efferent arteriole.

• It is a non-energy using method (ATP).

• Other capillary membranes are 1,000 times more permeable to water and solutes than the filtering membrane.

Reabsorption

The reabsorption process takes place mostly in the renal tubules (the proximal convoluted tubule (PCT), a loop of Henle, and the distal convoluted tubule (DCT). This is where 99% of the filtrate is produced. It refers to the return of substances from the filtrate (in the nephron) to the blood circulation (of peritubular capillaries). Urea, uric acid, and creatinine are nitrogenous wastes that are not reabsorbed. This is an active as well as a passive process.

• The Proximal Convoluted Tubule (PCT): The proximal convoluted tubules are where the majority of reabsorption takes place. The glucose in the filtrate is reabsorbed into the blood, most of which occurs in the PCT. The Na⁺-K⁺ pump, which employs ATP, actively transports most of the sodium cations (65%) from the proximal convoluted tubules back into circulation. Amino acids and vitamins are reabsorbed into the bloodstream from the PCT.

• Loop of Henle: The remaining water is reabsorbed by the descending limb. From the ascending limb, sodium ions and chloride ions are reabsorbed.

• The Distal Convoluted Tubule (DCT):  DCT is capable of selective reabsorption of chemicals that have remained in the filtrate. It is dependent on the body's present needs. Hormones such as aldosterone regulate it. Also, aldosterone regulates sodium ions’ reabsorption in DCT.   

Tubular Reabsorption in The Nephron

Secretion

• It includes tubular secretion. It entails adding substances to the filtrate obtained from the blood in peritubular capillaries (by active transport). 

• Any remaining nitrogenous wastes, some medications, and H⁺ ions are the main materials secreted to maintain the pH of the blood.

• Tubular cells secrete hydrogen ions, potassium ions, and other ions into the filtrate. 

• As Na⁺ is reabsorbed, the potassium (K⁺) cation is released into DCT. The hormone aldosterone regulates K⁺ secretion. 

• The pH of the blood rises as H⁺ ions are released into the filtrate (it becomes less acidic). 

• The secretion also removes urea, uric acid, ammonium ions, and certain medicines from the blood.

• Urine is the result of the fluid entering the collecting duct.

Tubular Secretion in Nephron

Mechanism of Urine Formation Diagram

The following diagram provides a clear understanding of the mechanism of urine formation and the various steps involved in it.

Urine Formation Steps

Mechanism of Urine Formation Flow Chart

The following flowchart explains the urine formation mechanism in a step-by-step manner.

Urine Formation - Flow Chart 

 Countercurrent Mechanism of Urine Formation

• The objective of countercurrent processes is to build and maintain an osmotic gradient of renal interstitial fluid from the renal cortex to the medulla, allowing the kidney to change the concentration of urine. 

• In countercurrent mechanisms, the juxtamedullary type of nephron is engaged.

• The countercurrent multiplier, or countercurrent mechanism, is used by the nephrons of the mammalian urinary system to reduce the amount of water in urine by which concentrated urine is formed in the kidneys. The Loop of Henle works as a countercurrent multiplier, while the Vasa Recta maintains the gradient and acts as a countercurrent exchanger.

• Countercurrent multiplication is the process by which countercurrent flow creates a steadily growing osmotic gradient in the interstitial fluid of the renal medulla. 

• The nephrons that produce concentrated urine run the length of the kidney, from the cortex to the medulla, and are carried by the vasa recta. 

• The filtrate travels in opposing ways into the two limbs of Henle's loop. Hence, the flow of blood cells in the vasa recta is similarly in opposite directions. Both the descending limb, which transports tubular fluid into the medulla and the ascending limb, which carry tubular fluid in the opposite direction, have countercurrent flow. 

Solved Problems

Q1. Which of the following structures constitutes the countercurrent mechanism?

1. Henle’s loop and glomerulus

2. PCT and DCT

3. Henle’s loop and vasa recta

4. Vasa recta and collecting 

Ans: (c)

The countercurrent mechanism's purpose is to produce urine with a different concentration than normal urine. The body prefers urine that is more concentrated than plasma to conserve water. The body expels excess water, and the osmolarity of urine will be lower than that of plasma. The Henle's loop of the nephron is a U-shaped segment. In the two arms of the tube, blood flows in opposite directions, generating countercurrents. In the renal medulla, the Vasa Recta is an efferent arteriole that forms a capillary network around the tubules. It is U-shaped and travels in the same direction as Henle's loop.

Q2. Which of the following is not passed on to the lumen of Bowman’s capsule during glomerular filtration?

• Fat molecules

• Glucose

• Water

• Proteins

Ans: (d)

Glomerular filtration occurs in the glomerulus of the nephron with blood flowing via the capillaries of the glomerulus to the filtrate in the glomerular capsule (Bowman's capsule). During glomerular filtration, proteins are not transmitted into the lumen of the Bowman's capsule as the blood is filtered so finely by the podocytes of the Bowman's capsule, that practically all of the plasma constituents except the proteins pass into the lumen of the Bowman's capsule. The glomerular filtration pressure is equal to the fluid pressure in the Bowman's space and osmotic force, which is owing to protein in plasma, which are the forces opposing filtration.

Conclusion

This article has given all the necessary details about the mechanism of urine formation with respect to the NEET syllabus. The questions added at the end of the article are frequently asked questions and are important from the exam point of view. Thus, the student can have a clearer understanding of the questions that might appear in the exam. Download the mechanism of urine formation PDF from our website to prepare well.